The present invention relates to a grain-oriented silicon steel sheet produced by forming tension-creating insulating coating films on a final annealed grain-oriented silicon steel sheet prepared by deliberately preventing the formation of inorganic mineral films composed of forsterite (Mg2SiO4) and so on and, further, smoothing the surfaces to the extent of showing specular gloss, and a method for producing the steel sheet.
A grain-oriented silicon steel sheet is widely used as a material for magnet cores and, for minimizing energy loss in particular, a silicon steel sheet having a small core loss is required. It is effective to impose a tension on a steel sheet to reduce core loss. For this reason, it has been a common practice to create a tension in a steel sheet and to reduce a core loss by forming coating films consisting of a material having a smaller thermal expansion coefficient than that of the steel sheet at a high temperature. A film of a forsterite type formed through the reaction of oxides, on a steel sheet surface, with an annealing separator in a final annealing process creates a tension in the steel sheet, and the adhesiveness of the film is excellent.
Japanese Unexamined Patent Publication No. S48-39338 discloses that the formation of insulating coating films by coating the surfaces of a steel sheet with a coating liquid mainly consisting of colloidal silica and phosphate and baking it has a significant effect on creating a tension in the steel sheet and is effective in reducing the core loss.
Therefore, the method of keeping the films of a forsterite type formed in a final annealing process and then forming insulating coating films mainly consisting of phosphate is generally employed as a method for producing a grain-oriented silicon steel sheet.
In recent years, it has been clarified that the disordered interfacial structure of a forsterite type film and a base metal somewhat reduces the effect of a coating film tension on improving a core loss. In view of this, a technology has been developed which attempts to further reduce a core loss by forming anew tension-creating coating films after removing the forsterite type films formed in a final annealing process and/or applying a mirror-finish further, as disclosed in Japanese Unexamined Patent Publication No. S49-96920, for example.
However, although said insulating coating film has an appreciable adhesiveness when it is formed on a film mainly composed of forsterite, it has an insufficient adhesiveness when it is formed after removing a forsterite type film or when a forsterite type film is intentionally prevented from forming in a final annealing process. When a forsterite type coating film is removed, in particular, it is necessary to secure a desired tension only with a tension-creating insulating coating film formed by coating a steel sheet surface with a coating liquid, and, therefore, it is necessary to make the insulating coating film thicker and a stronger adhesiveness is required. For this reason, by a conventional method of forming a coating film, it has been difficult to realize a coating film-induced tension high enough for making the best of the mirror finishing of a steel sheet surface and, at the same time, to secure the high adhesiveness of the coating film and, consequently, the core loss has not been reduced sufficiently. In view of this situation, the methods of forming oxide films on the surfaces of a final annealed grain-oriented silicon steel sheet prior to the formation of the tension-creating insulating coating films were disclosed, for example, in Japanese Unexamined Patent Publication Nos. S60-131976, H6-184762, H7-278833, H8-191010 and H9-078252, as the technologies for securing the adhesiveness of the tension-creating insulating coating films.
The method disclosed in Japanese Unexamined Patent Publication No. S60-131976 is a method of internally oxidizing the vicinity of the surfaces of a final annealed grain-oriented silicon steel sheet after mirror-finishing the steel sheet, for the purpose of improving the adhesiveness of the tension-creating coating films by the internally oxidized layers and, thus, compensating for the deterioration of the core loss resulting from the internal oxidation, namely the deterioration of specular gloss, with the increase in the tension brought about by the improved adhesiveness of the coating films.
The method disclosed in Japanese Unexamined Patent Publication No. H6-184762 is a method of securing the adhesiveness between each of tension-creating insulating coating films and a steel sheet by the effect of external oxidation type oxide films formed on the steel sheet surfaces by subjecting a final annealed grain-oriented silicon steel sheet conditioned into a mirror finish or the like to annealing in a prescribed atmosphere at each of the prescribed temperatures.
The technology disclosed in Japanese Unexamined Patent Publication No. H7-278833 is a technology for preventing the oxidation of a steel sheet, namely the deterioration of specular gloss, from occurring during the formation of crystalline tension-creating insulating coating films, when the tension-creating insulating coating films are in a crystalline state, by forming basic coating films composed of amorphous oxides beforehand on the surfaces of a final annealed grain-oriented silicon steel sheet free of inorganic mineral films. The method disclosed in Japanese Unexamined Patent Publication No. H8-191010 is a method of reducing a core loss by forming crystalline fayalite on the surfaces of a final annealed grain-oriented silicon steel sheet cleaned of non-metallic substances and utilizing the tension-creating and adhesiveness-improving effects of the fayalite crystals. The method disclosed in Japanese Unexamined Patent Publication No. H9-078252 is a method of securing the adhesiveness of tension-creating coating films and, at the same time, realizing a good core loss by controlling the amount of basic silica layers formed on the surfaces of a finish-annealed grain-oriented silicon steel sheet free of inorganic mineral films to 100 mg/m2 or less.
However, while it has been possible to realize the effects of improving the adhesiveness of coating films and reducing a core loss to an appreciable extent by forming oxide films on the surfaces of a grain-oriented silicon steel sheet free of an inorganic materials through the application of said technologies, the adhesiveness of the tension-creating insulating coating films has not been perfectly satisfactory. The present invention, which solves the above problems, is a method of forming tension-creating insulating coating films having a sufficient adhesiveness to a final annealed grain-oriented silicon steel sheet free of inorganic mineral coating films.
The gist of the present invention is as follows:
(1) A grain-oriented silicon steel sheet excellent in adhesiveness to tension-creating insulating coating films formed on the grain-oriented silicon steel sheet produced by removing inorganic mineral films composed of forsterite, and so on, by pickling or the like or by deliberately preventing the formation thereof, characterized by: having, at the interface between each of the tension-creating insulating coating films and the steel sheet, an external oxidation type membranous oxide film of 2 to 500 nm in average thickness mainly composed of amorphous silica and/or a mixed oxide film consisting of an external oxidation type membranous oxide film of 2 to 500 nm in average thickness mainly composed of amorphous silica and particulate oxides mainly composed of amorphous silica: and satisfying any one or more of the following requirements A to E;
A. that the percentage of said particulate oxides to said membranous oxide film is 2% or more in terms of area percentage at a cross-section;
B. that the percentage of oxides composed of one or more elements selected from among Fe, Al, Ti, Mn and Cr in said membranous oxide film is 50% or less in terms of area percentage at a cross-section;
C. that the percentage in voids to said membranous oxide film is 30% or less in terms of area percentage at a cross-section;
D. that the percentage of metallic iron in said membranous oxide film is 30% or less in terms of area percentage at a cross-section; and
E. that the average thickness of low-density layers is 30% or less of the total thickness of said membranous oxide film when they are evaluated in terms of the ratio between elastic scattering strength and inelastic scattering strength measured by electron energy loss spectroscopy.
(2) A grain-oriented silicon steel sheet excellent in adhesiveness to tension-creating insulating coating films according to the (1), characterized in that the tension-creating insulating coating films are the coating films formed by baking an application liquid mainly composed of phosphate and colloidal silica and/or an application liquid mainly composed of alumina sol and boric acid.
(3) A method for producing a grain-oriented silicon steel sheet excellent in adhesiveness to tension-creating insulating coating films formed by, in advance of the formation of the tension-creating insulating coating films: annealing a final annealed grain-oriented silicon steel sheet produced by removing the inorganic mineral coating films composed of forsterite, and so on, by pickling or the like or by deliberately preventing the formation thereof in a low-oxidizing atmosphere to form oxides on the surfaces thereof; then applying a liquid for forming the tension-creating insulating coating films; and baking the application liquid: characterized by satisfying any one or more of the following requirements A to E:
A. to form particulate oxides mainly composed of amorphous silica in addition to external oxidation type membranous oxide films of 2 to 500 nm in average thickness mainly composed of amorphous silica by imposing micro-strains and/or forming micro-roughnesses on the surfaces of the steel sheet prior to the annealing in a low-oxidizing atmosphere for forming the oxides, and then annealing the steel sheet in a low-oxidizing atmosphere at a temperature from 600 to 1,150xc2x0 C.;
B. to control the percentage of oxides composed of one or more elements selected from among Fe, Al, Ti, Mn and Cr in the external oxidation type oxide films mainly composed of amorphous silica to 50% or less in terms of area percentage at a section by controlling the heating rate to 10 to 500xc2x0 C./sec. in a heating temperature range from 200 to 1,150xc2x0 C., during the annealing process in a low-oxidizing atmosphere for forming the external oxidation type membranous oxide films and the particulate oxides;
C. to control the percentage of voids in the external oxidation type oxide films mainly composed of amorphous silica to 30% or less in terms of area percentage at a section by controlling the cooling rate to 100xc2x0 C./sec. or less in a cooling temperature range from 1,150 to 200xc2x0 C., during the annealing process in a low-oxidizing atmosphere for forming the external oxidation type oxide films and the particulate oxides;
D. to control the percentage of metallic iron in the external oxidation type oxide films mainly composed of amorphous silica to 30% or less in terms of area percentage at a section by controlling the dew point of the cooling atmosphere to 60xc2x0 C. or lower in a cooling temperature range from 1,150 to 200xc2x0 C., during the annealing process in a low-oxidizing atmosphere for forming the external oxidation type oxide films and the particulate oxides; and
E. to control the average thickness of low-density layers to 30% or less of the total thickness of the external oxidation type oxide films mainly composed of amorphous silica, when they are evaluated in terms of the ratio between elastic scattering strength and inelastic scattering strength measured by electron energy loss spectroscopy, by controlling the time during which the application liquid for forming the tension-creating insulating coating films and the steel sheet with the amorphous silica contact each other to 20 sec. or less, in the temperature range of 100xc2x0 C. or lower, in the method of forming the tension-creating insulating coating films by applying the liquid for forming the tension-creating insulating coating films and baking the application liquid.
(4) A method for producing a grain-oriented silicon steel sheet excellent in adhesiveness to tension-creating insulating coating films according to the item (3), characterized by baking an application liquid mainly composed of phosphate and colloidal silica and/or an application liquid mainly composed of alumina sol and boric acid.